This invention relates generally to a total internal reflection knife edge utilized in a raster output scanning system to improve the accuracy of the start of scan detection and more particularly, to a transparent optical clement which is used as a knife edge to reflect a light beam at certain locations.
Typically, a fiber optic element is used to detect the start of scan in order to trigger the electronics to send the video stream at a precise time which corresponds to a precise location for the light exposure on a desired media. This is achieved by placing the entrance surface of a fiber optic element in the path of the scanning light beam. When the light beam passes the entrance surface of a fiber optic element, the light beam enters the fiber optic element and it is transferred to the detecting circuit. In response to the detection of the light beam, the detecting circuit generates a start of the scan pulse. In this method, in order to block the light beam from entering the fiber optic at certain locations a knife edge is used. Blocking the light beam from certain locations improves the accuracy of the detection of the start of scan.
For example, referring to FIG. 1, there is shown a prior art optical system 10 which utilizes a knife edge 12 to block a light beam 14 (shown by dashed lines) and to allow the light beam 16 (shown by the solid lines) to enter a fiber optic element 18. In FIG. 1 since the light beam is scanning a scan line, at each given time, it has a different position such as spot S and spot S'. Spot S is the source of the light for the light beam 16 and spot S' is the source of the light for the light beam 14. Therefore, if spot S is at the start of the scan, then spot S' is slightly away from the start of the scan. In this case, if the knife edge 12 does not exist, both light beams 14 and 16 from the spots S' and S respectively will enter the fiber optic element 18 and therefore, the detection of the start of the scan will not be accurate.
The knife edge 12 is placed prior to the entrance surface 19 of the fiber optic element 18 in order to block the light beam 14 and allow the light beam 16 to enter the fiber optic element 18. It should be noted that between spot S and S' there are infinite infinitesimal position changes of the spot S. However, the knife edge 12 blocks the light beam from a certain number of locations to reduce the inaccuracy in the detection of the start of scan.
Typically, in order to block the light beam from entering the fiber optic element, a layer of reflective material is applied to the knife edge 12 on the surface which receives a light beam. The reflective material causes the knife edge to reflect back the received light beam. An alternative is to build the knife edge with a light absorbing material.
It should be noted that in order to direct the scanning light beam from a light source onto a fiber optic, certain optical elements such as a mirror might be used. However in FIG. 1, for the purpose of simplicity, those elements are not shown.
In addition, in the optical system 10 of the prior art, the knife edge is placed at a distance d from the light source S in order to focus a converging light beam on the knife edge. However, referring to FIG. 2, in order to reduce the distance between the light source S and the knife edge 12 a focusing lens 20 can be placed between the light source and the knife edge 12.
It is an object of this invention to provide a transparent optical element which perform as a focusing lens and a knife edge.